Provided are an apparatus and method for converting a low-resolution depth image to a depth image having a resolution identical to a resolution of a high-resolution color image. The depth image conversion apparatus may generate a discrete depth image by quantizing a depth value of an up-sampled depth image, estimate a high-resolution discrete depth image by optimizing an objective functions of the discrete depth image based on the high-resolution color image and an up-sampled depth border, and convert the up-sampled depth image to a high-resolution depth image by filtering the up-sampled depth image when a difference between discrete depth values of neighboring pixels in the high-resolution discrete depth image is less than a predetermined threshold value.